Electrons and other charged particles process spins as one of their intrinsic particle properties and such a spin is associated with a spin angular momentum. A spin of an electron has two distinctive spin states. Electrons in an electrical current may be unpolarized by having the equal probabilities in the two spin states. The electrons in an electrical current are spin polarized by having more electrons in one spin state than electrons in the other spin state. A spin-polarized current can be achieved by manipulating the spin population via various methods, e.g., by passing the current through a magnetic layer having a particular magnetization. In various magnetic microstructures, a spin-polarized current can be directed into a magnetic layer to cause transfer of the angular momenta of the spin-polarized electrons to the magnetic layer and this transfer can lead to exertion of a spin-transfer torque on the local magnetic moments in the magnetic layer and precession of the magnetic moments in the magnetic layer. Under a proper condition, this spin-transfer torque can cause a flip or switch of the direction of the magnetization of the magnetic layer.
The above spin-transfer torque (STT) effect can be used for various applications including STT magnetic random access memory (MRAM) circuits and devices. For example, as illustrated in FIG. 1, a STT-MRAM circuit can include a magnetic tunnel junction (MTJ) as a magnetoresistive element formed of two or more thin film ferromagnetic layers or electrodes, which are usually referred to as the free magnetic layer (FL) having a magnetic moment that can be switched or changed and the pinned magnetic layer (PL) whose magnetic moment is fixed in direction. The free magnetic layer (FL) and the pinned magnetic layer (PL) are separated by an insulating barrier layer (e.g., a MgO layer) that is sufficiently thin to allow electrons to transit through the barrier layer via quantum mechanical tunneling when an electrical bias voltage is applied between the electrodes. The electrical resistance across the MTJ depends upon the relative magnetic orientations of the PL and FL layers. The magnetic moment of the FL can be switched between two stable orientations in the FL. The resistance across the MTJ exhibits two different values under the two relative magnetic orientations of the PL and FL layers, which can be used to represent two binary states “1” and “0” for binary data storage, or, alternatively, for binary logic applications. The magnetoresistance of this element is used to read out this binary information from the memory or logic cell.
In various STT-MRAM and other circuits, the MTJ is a 2-terminal MTJ circuit that directs a current from one terminal through the tunnel barrier to the other terminal. FIG. 1 further illustrates this 2-terminal circuit configuration where a 2-terminal control circuit is coupled to the terminals on two sides of the MTJ. In a write operation, the 2-terminal control circuit sends a sufficiently large write current in a selected current flow direction from one terminal through the barrier layer to the other terminal through the barrier layer to set the magnetic orientation of the free layer relative to the reference layer representing a desired binary state. In a read operation, the 2-terminal control circuit uses the same two terminals to send a read current which is less than the large write current through the barrier layer to measure the resistance across the MTJ corresponding to a stored bit under a particular relative magnetic orientation of the PL and FL layers.